Bosque de Chapultepec (commonly called Chapultepec Park) is considered the lungs of Mexico City. It houses a castle, a zoo, a cultural center, multiple lakes, vendors, museums, presidential palaces, and pretty much anything you could want in a city. At 1,695 acres, you can spend days here wandering around, but your best bet is to make a list of what you want to see the most and head to those spots.